Refineries typically process a blend of crude oils rather than a single crude oil to ensure that an optimum product mix as required by markets, which varies from time to time, can be obtained at minimum costs. Increasingly, refineries are looking for ways to co-process heavy crude oils with light crude oils. Heavy crude oils have a high amount of paraffin or asphaltene or both. When there is a high paraffin content, it results in high viscosity and high pour point making transportation difficult. On the other hand, a high asphaltene content causes precipitation, flocculation, instability, and incompatibility related problems during processing. Therefore, increasing co-processing of heavy oil components is a real challenge.
To increase the heavy oil content in the mix of crude oils processed and also for suitable oil selection for processing, refiners encounter five common problems on day-to-day operation viz. (i) Incompatibility/Stability issues when the crude oils are blended, (ii) high viscosity of the blend, (iii) high pour point of the blend, (iv) high sulphur content in the blend, and (v) high vacuum residue yields upon processing. While selection of crude oils and blends is done, various simulations and correlations are used to ensure viscosity, pour point, distillation and residue yields, and sulphur content is appropriately managed. However, incompatibility of crude oils and blends is still a grey area, which is currently being managed by previous operational experience and compatibility test results received from oil testing laboratories. Incompatibility is a serious problem which causes asphaltene precipitation and severely affects various equipment viz. tanks, heat exchangers, separators, columns pumps, etc., and in the absence of viable models to manage incompatibility, refining operations and costs can get severely affected.
There are various test methods reported and being practiced at laboratories in order to study the stability of crude oils and blends viz. colloidal instability index (CII), colloidal stability index (CSI), Stankiewicz plot (SP), qualitative-quantitative analysis (QQA), stability cross plot (SCP), Heithaus parameter (or parameter P), Heptane Dilution (HD), toluene equivalence (TE), spot test and oil compatibility model (OCM). Some of the tests are correlation based, which requires Saturates, Aromatics, Resins and Asphaltene (SARA) analysis data to predict the stability of crude oils. OCM, TE and P parameter methods require asphaltene precipitation data when solvents like heptane or toluene are added in different ratio to estimate the stability of crude oils. Thus, these tests require samples of the crude oils and blends to be chemically tested in the laboratory before a decision can be made for purchasing and processing the crude oils. Moreover, as the test results may vary depending on the test method used, methods based on one or two tests are incapable of accurately predicting compatibility. In addition to this, models reported in the literature are not applicable to crude oils which have a low amount of asphaltene (≤0.5 wt %) and high amount of saturates. As a result, more than three to four tests are needed to confirm the compatibility of crude oils and blends. While these methods are comprehensive, they are experiment based and time consuming. By the time results are available, suboptimal crude oil blends are processed or asphaltene precipitation problems are created in the refineries, which requires substantial efforts and price to resolve.
In the past, various research groups have attempted to develop methods and correlation models to predict asphaltene precipitation and crude oil stability and compatibility. Insolubility number (IN) and solubility blending number (SBN) have been recognized as important parameters to predict asphaltene precipitation Higher the SBN number, there is less possibility for incompatibility. Sollaimany and Bayandori studied the asphaltene deposition behavior. According to their results, CSI such as the (Aromatics+Resin)/(Asphaltene+Saturates) ratio and (Aromatics+Resin)/Asphaltene ratio are less significant for asphaltene stability rather structural parameters of asphaltenes and resins are important. Asomaning conducted various tests such as spot test, CII, Asphaltene/Resin ratio, and solvent method with near-infrared spectroscopy to predict the asphaltene stability in crude oils. CII and solvent titration method prediction results found closer to the field deposition results. However, some of the papers concluded that CII is not an appropriate method for prediction of asphaltene precipitation when asphaltene content is low and saturate content is high. Guzman R., et. al., have extensively reviewed different methods and concluded that CII and SP methods were predicts unsatisfactory results, on the other hand, QQA and SCP predicts better for asphaltene stability in crude oils. Stratiev et al. have correlated asphaltene solubility with the asphaltene hydrogen content and the oil solubility power correlated well with the oil saturate content. Higher asphaltene aromaticity help in solubilizing asphaltene compounds. They also found that some of the physical parameters such as specific gravity, Conradson carbon, and viscosity are able to predict the hydrogen, saturates, and asphaltenes content of vacuum residual oils. Similarly, there are few tests reported to predict asphaltene precipitation based on experimental physical properties data. Jose et. al., predicted incompatibility of blends of crude oils with liquid hydrocarbon from dynamic viscosity and density. They observed that increase in viscosity and density of the crude oil blends mixed with liquid hydrocarbon increases tendency of asphaltene aggregation. Incompatible region determined by viscosity data is more superior to density data. Thus, while there are multiple different approaches studied for determining compatibility of crude oils in blends, the methods are usually time consuming and not fully reliable.